President Bush, the ultimate conviction politician, showed that his elongated time as a lame duck president with the lowest ratings in history has given him a chance to reflect on his contributions to the world. Talking to the Times on board Air Force One, the president bemoaned his image as a warmonger suggesting that "in retrospect I could have used a different tone, a different rhetoric". Yet it is the reality of his policies, not his rhetoric, which will be his most enduring legacy.

Nowhere is this more disturbing than the US descent into barbarism. Extraordinary rendition, Abu Ghraib, Guantánamo, waterboarding – over the last seven years a new lexicon has emerged that illustrates how "fighting terror" has warped America's moral compass. Alex Gibney's documentary of this descent, Taxi to the Dark Side, is a well balanced and researched indictment of the failure of the Bush administration's "command responsibility" and its tragic consequences "on the ground".

Much like the winter soldiers who spoke out against Vietnam, Taxi to the Dark Side includes the voices of those who patriotism cannot be questioned, the soldiers themselves. The documentary's underlying message is that the abuses that have emerged over the years are not simply the work of "bad apples" but rather are the product of the policies Dick Cheney outlined five days after 9/11:

We also have to work, though, sort of the dark side, if you will. We've got to spend time in the shadows in the intelligence world. A lot of what needs to be done here will have to be done quietly, without any discussion, using sources and methods that are available to our intelligence agencies, if we're going to be successful. That's the world these folks operate in, and so it's going to be vital for us to use any means at our disposal, basically, to achieve our objective.

The professionally trained killers that the US sent to fight its global wars therefore travelled with only a loose code of conduct informed by the use of "any means at our disposal". When an administration at first ignores and then proceeds to selectively interpret the Geneva conventions, abuses are certain to occur.

Indeed the "gloves are off" mentality that followed 9/11 is shown to have led directly to the killings of prisoners in detention. Taxi to the Dark Side places the macro-impact of US policies in the context of the micro-impact on one man – Dilwar. This 22-year-old taxi driver found himself arrested by Afghan militiamen who were working with the US (the same militiamen were later found to have been the ones firing rockets at the US base – a worrying phenomena when allied with the statistic that only 7% of those held at Guantánamo were arrested by US forces). Dilwar died as a result of beatings he received during his interrogations. The gloves were off to such an extent that his legs were pulped and if he'd survived would have had to have been amputated. One of his interrogators explained that it was the "us against them" mentality that drove this level of hostility towards detainees.

Lawyers like John Yoo were then used to provide a labyrinth of legal buffers that would justify America's new hardline approach to bringing terrorists to account – defining torture as pain equivalent to "death or organ failure". President Bush announced in his 2003 state of the union address that "one by one, the terrorists are learning the meaning of American justice". Yet this justice was to be dispensed without trials in Guantánamo, a physical loophole location that became what the documentary called a "laboratory for behavioral techniques" including sensory deprivation (blindfolds, earmuffs, forced sleep loss, stress positions) and targeting "detainee specific phobias" (hence the use of nudity and dogs in Abu Ghraib – as Arab men are supposedly susceptible to sexual humiliation and canines).

Professor Alfred McCoy explained the increase in American use of torture as a continuation of over 50 years of "undetected cancer inside the US intelligence community". This trend has been reinforced post 9/11 by popular culture that has grown into a constituency that condones torture. This is the "Jack Bauer effect" – that sells a hypothetical doomsday scenario as almost standard operating procedure and the only protection from the mushroom-cloud alternative.

Taxi to the Dark Side is a sobering lesson in the dangers of unchecked power been given to those who are at the sharp end of our wars. Dostoevsky famously once said that "the degree of civilisation in a society can be judged by entering its prisons". If America is to rediscover its moral compass it must ensure proper justice for all.